Polymeric materials contain low molecular weight impurities that come from a variety of sources. For example, the process for the manufacture of polymeric materials can result in impurities, such as the presence of residual monomers or non-polymerizable impurities present in the monomers, short chain oligomers and products resulting from the decomposition of products added to the polymerization mixture to initiate/catalyze/or accelerate the reaction. Sometimes, organic solvents are also added to the polymerization mixture to facilitate processabilty (e.g., to lower the viscosity) or to impart certain morphological characteristics to the final polymeric material product. Furthermore, low molecular weight additives are sometimes added to polymeric materials to improve their performance in use. These additives include plasticizers, anti-oxidants, light stabilizers, and the like.
Once the polymeric raw material has been incorporated or formed into an article of manufacture, these impurities can present a problem, ranging from minor to serious, particularly depending upon what the formed article is. These impurities can diffuse out of the polymer phase into the various streams with which the polymer enters into contact. This possible contamination is of particularly concern for articles used in medical applications, such as for example, in bottles and drums to be used as packaging for ultrapure chemicals or reagents, beakers, funnels, separatory funnels, reactors and chambers, plastic columns, pipes, tubing, connectors, screens and adapters to be used in laboratory equipment and/instruments, blood bags, oxygen tubing, intravenous equipment, enteral food or parental nutrition bags, implants, food packaging, and toys and the like. For instance, concerns about the use of phthalates as plasticizers in PVC have recently been raised; C&E News, Aug. 7, 2000, 52-54; Science News, 158, 152-154, September 2000. Contamination of streams coming in contact with a plastic article is also of concern in analytical applications or medical research, particularly in “throw-away” once-through articles that typically receive little conditioning before use. For example, it is known that the light stabilizer Tinuvin 770 that elutes from polypropylene tubes is a potent L-type Ca2+ channel blocker; H. S. Glossman et al., Proc. Natl. Acad. Sci. USA 90, 9523-9527 (1993). Other examples of such articles or devices where diffusion of impurities out of the plastic article or device can present significant problems are bottles and drums to be used as packaging for ultrapure chemicals or reagents, beakers, funnels, separatory funnels, reactors and chambers, plastic columns, pipes, tubing, connectors, screens and adapters to be used in laboratory equipment and/instruments, blood bags, oxygen tubing, intravenous equipment, enteral food or parental nutrition bags, implants, food packaging, and toys.
Methods for removing low molecular weight impurities from polymeric materials have been recently reviewed in H. J. Vandenburg et al., Analyst 122, 101R-115R (1997). One such method consists in dissolving the polymer and its impurities in a suitable solvent, precipitation of the polymer and analysis of the content of the supernatant. This method has been used to analyze residual monomers and oligomers in polyethylene terephtalate, plasitcizers or stabilizers in poly(vinyl chloride), additives in polyethylene, polypropylene, polyamides, polycarbonates and polysulfones. Although this dissolution approach provides fast and complete separation of low molecular weight components from polymeric materials, it is apparent that it is not applicable when the purpose of the extraction is to produce a cleaner polymeric article in its original physical form (e.g., pellets for molding), nor applicable to non soluble (e.g., crosslinked) polymers.
Low molecular weight components or impurities can be extracted form polymers, in a batch equilibration or continuous extraction mode without irreversibly affecting the physical form of the latter by contacting said polymers with solvents that do not cause it to dissolve. Although numerous examples of this method have been discussed, including the extraction of additives from polyethylene and polypropylene, these methods relate primarily to the analysis of additives as opposed to residual monomers or oligomers.
All the extraction methods described above are analytical procedures aimed at identifying and quantifying the amount of low molecular weight contaminants present in polymeric materials and not for providing ultrapure polymeric materials and articles manufactured therefrom.
Although solvent extraction can be an effective cleaning procedure, a certain number of problems are also associated with it. The solvent extraction process operation usually requires the use of rather large quantities of solvents. First, this has a negative impact on cost, both in solvent purchase and disposal or recycling of the solvent. Secondly, depending on the nature of the solvent used for the procedure, flammability, VOC emission or other environmental issues can come into play. Additionally, lengthy drying procedures are usually required to fully remove from the polymeric material the very substance that is used to clean it, resulting in increased operational complexity and higher costs.
A number of the issues associated with cleaning up polymeric materials using organic solvents can be avoided by using aqueous streams or steam. However, because the impurities that one is attempting to remove are often hydrophobic in nature, and because water is often not a particularly good swelling agent for the polymer phase at hand, their removal with water is not particularly efficient. Steam can be more effective but, with most polymeric materials, one is limited in the maximum temperature that can be used by either degradation of polymer morphology or the onset of chemical decomposition. Although the extraction of impurities from the bulk of finely divided polymers by aqueous solutions containing peroxides has been disclosed in European Patent Publication No. EP 0 652 283, more often than not, aqueous based procedures are used to clean the surface of polymeric materials rather than to extract contaminants from inside the polymer phase. The cleaning of polymer surfaces by various aqueous based streams has been reviewed by B. Weiss, Oberflasche JOT 26(9), 27-34 (1989).
Whether water or steam is used for the removal of contaminants, the cleaned-up polymeric material is typically recovered wet with water, and, if a final product in the dry form is desired, the drying procedures are even more involved and energy intensive than in the case of organic solvents.
Removal and recovery of unreacted monomers from latex coagulums using hot water have been described in U.S. Pat. No. 3,954,910, but the approach appears limited to hydrophilic monomers such as monounsaturated nitrites.
Accelerated extraction of additives from polymeric material using organic solvents in the supercritical domain for analytical purposes has been described in H. J. Vandenburg et al., Anal Chem., 70, 1943-1948 (1998). Although it was demonstrated that accelerated extraction of stabilizers could be achieved relative to conventional extraction methods, the same problems discussed above in relation to conventional solvent extraction come into play here as well.
Furthermore, even though it has been possible to clean polymer materials using such solvent or aqueous processes, upon molding the polymeric materials into articles the polymeric material in the articles has been found to contain impurities such as oligomers or monomeric material produced as a result of the heat of the molding operation.
Simply for analytical purposes, supercritical CO2 has been used to remove monomers and low molecular weight oligomers from polymer such as nylon, poly alkylene terephtalates or polystyrene. In the particular case of polyethylene terephtalate, removal of low molecular weight by-products by supercritical CO2 during the polymerization process has been described—L. C. Burke et al., Polym. Mater. Sci. Eng. 74, 248-249 (1996). Except at very high temperatures, the addition of organic modifiers is required to achieve complete quantitative extraction. Supercritical extraction of stabilizing additives from articles made from PE and PP for analytical purposes is described as being possible with CO2 only—N. J. Cotton et al., J. Appl. Polym Sci. 48 (9), 1607-1619 (1993). The impact of various parameters on the efficiency of the additives extraction process has been studied—L. Xiamen et al., J. Microcolumn September 7 (4), 303-317 (1995); H. J. Vanderburg, et. al., Anal. Chem 70 (9), 1943-1948, (1998); F. Martial et al., Polym. Int. 48 (8), 299-306 (1999), L. Y. Zhou et al., J. Chrom. A 858 (2), 209-218 (1999). However, we are aware of no reports describing the quantitative extraction of monomers or oligomers specifically from polypropylene or polyethylene raw materials and then producing article of manufacture from the purified polymer without again producing oligomeric or monomeric impurities as a result of the molding operation of the article of manufacture, although one report describes extraction of “paraffins and olefins” from polyethylene for cytotoxicity testing, using supercritical CO2—J. H. Braybrook et al., Polymer International, vol. 27, pp. 157-164 (1992).
As in the case of the extraction of residual monomers and oligomers from nylon, polyalkylene terephtalates or polystyrene, organic additives or “modifiers” are sometimes added to supercritical CO2 to enhance the extraction of additives from polyethylene or polypropylene—A. Pinto and L. Taylor, J. Chrom A 811 (1+2), 163-170 (1998). The use of such modifiers has been applied to analyze for the presence of potential migrants from polypropylene in food contact applications—T. Buecherl et al., Dtsch. Lebensm.—Rundsch. 89 (3), 69-71(1993).
There remains need for a quantitative process for extraction of impurities from polymeric material such as polyolefins, e.g., polyethylene and polypropylene, and the ability to prepare ultrapure polymeric articles from the polymeric material without the reoccurrence of impurities, such as monomers or oligomers or short polymeric chains, when the polymeric articles are formed.